There's no support for tabs in Selenium as of now. A fairly recent answer by one of the Selenium developers explains it pretty clearly:
Selenium has no ability to switch tabs at the moment. Because of this
we force the browser to open links in new windows but since we are
able to switch windows we force the browser to take the approach. This
may be fixed in a later version.
Also, if you get away from IDE (or export your testcase to WebDriver) and get to a more powerful tool (any full-strength programming language binding for Selenium), you might use some kind of workaround:
https://stackoverflow.com/a/12730918/1273080 (Taps Ctrl+2 to get to the second tab etc.) However, this kind of workarounds is usually limited to a certain browser/OS/browser setting (tabs can be disabled).
That said, if you don't insist on using tabs and would use several browser windows instead, that's fairly easy in IDE as well as Selenium RC and WebDriver:
Selenium IDE: selectWindow
(you can spawn a new window by openWindow)
Selenium RC: selectWindow()
Selenium WebDriver: switchTo().window()